- Pgp is a 170 kDa plasma membrane glycoprotein that functions as a non-specific energy-dependent drug efflux pump. Pgp is expressed in a variety of normal human tissues, such as renal proximal tubules, capillary endothelial cells that comprise the blood-brain barrier, epithelial cells lining the bile canaliculi, bone marrow stem cells, and peripherial blood mononuclear cells.